1. Field of the Invention
This invention relates to a system for purifying the exhaust gases of diesel engines, comprising an exhaust gas line, which is connected to form two branch lines, which are adapted to be shut off in alternation by change-over valve means and one of which constitutes a filter line, which incorporates a soot filter and downstream of said soot filter is joined to the other branch line, which constitutes a by-pass line, whereby a common end line is formed. Said changeover valve means are operable in dependence on the load on the engine so that only the filter line will be open when the engine is operating under its full load or under a partial load in excess of a predetermined value.
2. Description of the Prior Art
The design of that system is based on the recognition that diesel soot particles are formed in diesel engines mainly during an operation of the engine under full load and in the upper partial load range and that particles which are contained in the exhaust gas during an operation under partial load will mainly consist of incompletely burnt hydrocarbons of the fuel and of the lubricating oil. Because the filter line which is opened only during an operation of the engine under its full load or under a partial load in excess of a predetermined value, the soot filter will not be effective unless the exhaust gas is at a sufficiently high temperature, which is sufficient for a self-regeneration of the soot filter and in that case there will be no need for separate means for igniting the soot which has been collected in the soot filter. But because a major part of the emission of particles will take place during an operation of the engine under a load in that high range, a relatively high proportion of the soot can be removed from the exhaust gas in that case. Tests have shown that 75 to 85 percent of the total amount of particles will be emitted by the engine during an operation in that high load range.
Such a system has already been disclosed in Published German Application 35 32 345 and in U.S. Pat. No. 3,937,015. But it will be understood that the exhaust gas will not be purified at all by that known system during an operation of the engine under relatively low partial loads so that the results are not entirely satisfactory.
It is known from U.S. Pat. No. 4,485,622 that a soot filter can be provided with a coating which ensures that the filter will also be effective as an oxidation-inducing catalyst for diesel soot. But such a filter cannot effect a purification to an optimum degree because owing to the surface coating the filter must have substantial dimensions, which will not be consistent with the requirements for filtration.